Committing hardware transactions that are about to run out of resource

ABSTRACT

A transactional memory system determines whether a hardware transaction can be salvaged. A processor of the transactional memory system begins execution of a transaction in a transactional memory environment. Based on detection that an amount of available resource for transactional execution is below a predetermined threshold level, the processor determines whether the transaction can be salvaged. Based on determining that the transaction can not be salvaged, the processor aborts the transaction. Based on determining the transaction can be salvaged, the processor performs a salvage operation, wherein the salvage operation comprises one or more of: determining that the transaction can be brought to a stable state without exceeding the amount of available resource for transactional execution, and bringing the transaction to a stable state; and determining that a resource can be made available, and making the resource available.

FIELD OF INVENTION

This disclosure relates generally to transactional execution, and morespecifically to determining whether hardware transactions that are aboutto run out of resource in transactional memory can be committed safely.

BACKGROUND

The number of central processing unit (CPU) cores on a chip and thenumber of CPU cores connected to a shared memory continues to growsignificantly to support growing workload capacity demand. Theincreasing number of CPUs cooperating to process the same workloads putsa significant burden on software scalability; for example, shared queuesor data-structures protected by traditional semaphores become hot spotsand lead to sub-linear n-way scaling curves. Traditionally, this hasbeen countered by implementing finer-grained locking in software, andwith lower latency/higher bandwidth interconnects in hardware.Implementing fine-grained locking to improve software scalability can bevery complicated and error-prone, and at today's CPU frequencies, thelatencies of hardware interconnects are limited by the physicaldimension of the chips and systems, and by the speed of light.

Implementations of hardware Transactional Memory (HTM, or in thisdiscussion, simply TM) have been introduced, wherein a group ofinstructions—called a transaction—operate in an atomic manner on a datastructure in memory, as viewed by other central processing units (CPUs)and the I/O subsystem (atomic operation is also known as “blockconcurrent” or “serialized” in other literature). The transactionexecutes optimistically without obtaining a lock, but may need to abortand retry the transaction execution if an operation, of the executingtransaction, on a memory location conflicts with another operation onthe same memory location. Previously, software transactional memoryimplementations have been proposed to support software TransactionalMemory (TM). However, hardware TM can provide improved performanceaspects and ease of use over software TM.

U.S. Patent Publication No. 2007/0028056 titled “Direct-Update SoftwareTransactional Memory” filed Jul. 29, 2005, incorporated herein byreference in its entirety, teaches a transactional memory programminginterface that allows a thread to directly and safely access one or moreshared memory locations within a transaction while maintaining controlstructures to manage memory accesses to those same locations by one ormore other concurrent threads. Each memory location accessed by thethread is associated with an enlistment record, and each threadmaintains a transaction log of its memory accesses. Within atransaction, a read operation is performed directly on the memorylocation and a write operation is attempted directly on the memorylocation, as opposed to some intermediate buffer. The thread can detectinconsistencies between the enlistment record of a memory location andthe thread's transaction log to determine whether the memory accesseswithin the transaction are not reliable and the transaction should bere-tried.

U.S. Patent Publication No. 2011/0119452 titled “Hybrid TransactionalMemory System (HybridTM) and Method” filed Nov. 6, 2009, incorporatedherein by reference in its entirety, teaches a computer processingsystem having memory and processing facilities for processing data witha computer program is a Hybrid Transactional Memory multiprocessorsystem with modules 1 . . . n coupled to a system physical memory array,I/O devices via a high speed interconnection element. A CPU isintegrated as in a multi-chip module with microprocessors which containor are coupled in the CPU module to an assist thread facility, as wellas a memory controller, cache controllers, cache memory, and othercomponents which form part of the CPU which connects to the high speedinterconnect which functions under the architecture and operating systemto interconnect elements of the computer system with physical memory,various I/O devices, and the other CPUs of the system. The currenthybrid transactional memory elements support for a transaction memorysystem that has a simple/cost effective hardware design that can dealwith limited hardware resources, yet one which has a transactionalfacility control logic providing for a backup assist thread that canstill allow transactions to reference existing libraries and allowsprogrammers to include calls to existing software libraries inside oftheir transaction, and which will not make a user code use a second lockbased solution.

SUMMARY

Embodiments of the present disclosure provide a method, computer system,and computer program product for a transactional memory system thatdetermines whether a hardware transaction can be salvaged. A processorof the transactional memory system begins execution of a transaction ina transactional memory environment. Based on detection that an amount ofavailable resource for transactional execution is below a predeterminedthreshold level, the processor determines whether the transaction can besalvaged. Based on determining that the transaction can not be salvaged,the processor aborts the transaction. Based on determining thetransaction can be salvaged, the processor performs a salvage operation,wherein the salvage operation comprises one or more of: determining thatthe transaction can be brought to a stable state without exceeding theamount of available resource for transactional execution, and bringingthe transaction to a stable state; and determining that a resource canbe made available, and making the resource available.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present disclosed embodiments areparticularly pointed out and distinctly claimed as examples in theclaims at the conclusion of the specification. The foregoing and otherobjects, features, and advantages of the disclosed embodiments areapparent from the following detailed description taken in conjunctionwith the accompanying drawings in which:

FIGS. 1 and 2 depict block diagrams of an example multi-coreTransactional Memory environment, in accordance with embodiments of thepresent disclosure.

FIG. 3 depicts a block diagram including example components of anexample CPU, in accordance with embodiments of the present disclosure.

FIG. 4 depicts a flow diagram illustrating an embodiment for passingcontrol of a transaction to an about-to-run-out-of-resource handler upondetection that the amount of available resource for transactionalexecution of the hardware transaction is below a predetermined thresholdlevel, in accordance with embodiments of the present disclosure.

FIG. 5 depicts a diagram illustrating an embodiment of the presentinvention.

FIG. 6 depicts a diagram illustrating an embodiment of the presentinvention.

FIG. 7 depicts a diagram illustrating an embodiment of the presentinvention.

FIG. 8 depicts a diagram illustrating an embodiment of the presentinvention.

FIG. 9 depicts a functional block diagram of a computer system, inaccordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

Historically, a computer system or processor had only a single processor(aka processing unit or central processing unit). The processor includedan instruction processing unit (IPU), a branch unit, a memory controlunit and the like. Such processors were capable of executing a singlethread of a program at a time. Operating systems were developed thatcould time-share a processor by dispatching a program to be executed onthe processor for a period of time, and then dispatching another programto be executed on the processor for another period of time. Astechnology evolved, memory subsystem caches were often added to theprocessor as well as complex dynamic address translation includingtranslation lookaside buffers (TLBs). The IPU itself was often referredto as a processor. As technology continued to evolve, an entireprocessor, could be packaged as a single semiconductor chip or die, sucha processor was referred to as a microprocessor. Then processors weredeveloped that incorporated multiple IPUs, such processors were oftenreferred to as multi-processors. Each such processor of amulti-processor computer system (processor) may include individual orshared caches, memory interfaces, system bus, address translationmechanism and the like. Virtual machine and instruction set architecture(ISA) emulators added a layer of software to a processor, that providedthe virtual machine with multiple “virtual processors” (aka processors)by time-slice usage of a single IPU in a single hardware processor. Astechnology further evolved, multi-threaded processors were developed,enabling a single hardware processor having a single multi-thread IPU toprovide a capability of simultaneously executing threads of differentprograms, thus, each thread of a multi-threaded processor appeared tothe operating system as a processor. As technology further evolved, itwas possible to put multiple processors (each having an IPU) on a singlesemiconductor chip or die. These processors were referred to processorcores or just cores. Thus, the terms such as processor, centralprocessing unit, processing unit, microprocessor, core, processor core,processor thread, and thread, for example, are often usedinterchangeably. Aspects of embodiments herein may be practiced by anyor all processors including those shown supra, without departing fromthe teachings herein. Wherein the term “thread” or “processor thread” isused herein, it is expected that particular advantage of the embodimentmay be had in a processor thread implementation.

Transaction Execution in Intel® Based Embodiments

In “Intel® Architecture Instruction Set Extensions ProgrammingReference” 319433-012A, February 2012, incorporated herein by referencein its entirety, Chapter 8 teaches, in part, that multithreadedapplications may take advantage of increasing numbers of CPU cores toachieve higher performance. However, the writing of multi-threadedapplications requires programmers to understand and take into accountdata sharing among the multiple threads. Access to shared data typicallyrequires synchronization mechanisms. These synchronization mechanismsare used to ensure that multiple threads update shared data byserializing operations that are applied to the shared data, oftenthrough the use of a critical section that is protected by a lock. Sinceserialization limits concurrency, programmers try to limit the overheaddue to synchronization.

Intel® Transactional Synchronization Extensions (Intel® TSX) allow aprocessor to dynamically determine whether threads need to be serializedthrough lock-protected critical sections, and to perform thatserialization only when required. This allows the processor to exposeand exploit concurrency that is hidden in an application because ofdynamically unnecessary synchronization.

With Intel TSX, programmer-specified code regions (also referred to as“transactional regions” or just “transactions”) are executedtransactionally. If the transactional execution completes successfully,then all memory operations performed within the transactional regionwill appear to have occurred instantaneously when viewed from otherprocessors. A processor makes the memory operations of the executedtransaction, performed within the transactional region, visible to otherprocessors only when a successful commit occurs, i.e., when thetransaction successfully completes execution. This process is oftenreferred to as an atomic commit.

Intel TSX provides two software interfaces to specify regions of codefor transactional execution. Hardware Lock Elision (HLE) is a legacycompatible instruction set extension (comprising the XACQUIRE andXRELEASE prefixes) to specify transactional regions. RestrictedTransactional Memory (RTM) is a new instruction set interface(comprising the XBEGIN, XEND, and XABORT instructions) for programmersto define transactional regions in a more flexible manner than thatpossible with HLE. HLE is for programmers who prefer the backwardcompatibility of the conventional mutual exclusion programming model andwould like to run HLE-enabled software on legacy hardware but would alsolike to take advantage of the new lock elision capabilities on hardwarewith HLE support. RTM is for programmers who prefer a flexible interfaceto the transactional execution hardware. In addition, Intel TSX alsoprovides an XTEST instruction. This instruction allows software to querywhether the logical processor is transactionally executing in atransactional region identified by either HLE or RTM.

Since a successful transactional execution ensures an atomic commit, theprocessor executes the code region optimistically without explicitsynchronization. If synchronization was unnecessary for that specificexecution, execution can commit without any cross-thread serialization.If the processor cannot commit atomically, then the optimistic executionfails. When this happens, the processor will roll back the execution, aprocess referred to as a transactional abort. On a transactional abort,the processor will discard all updates performed in the memory regionused by the transaction, restore architectural state to appear as if theoptimistic execution never occurred, and resume executionnon-transactionally.

A processor can perform a transactional abort for numerous reasons. Aprimary reason to abort a transaction is due to conflicting memoryaccesses between the transactionally executing logical processor andanother logical processor. Such conflicting memory accesses may preventa successful transactional execution. Memory addresses read from withina transactional region constitute the read-set of the transactionalregion and addresses written to within the transactional regionconstitute the write-set of the transactional region. Intel TSXmaintains the read- and write-sets at the granularity of a cache line. Aconflicting memory access occurs if another logical processor eitherreads a location that is part of the transactional region's write-set orwrites a location that is a part of either the read- or write-set of thetransactional region. A conflicting access typically means thatserialization is required for this code region. Since Intel TSX detectsdata conflicts at the granularity of a cache line, unrelated datalocations placed in the same cache line will be detected as conflictsthat result in transactional aborts. Transactional aborts may also occurdue to limited transactional resources. For example, the amount of dataaccessed in the region may exceed an implementation-specific capacity.Additionally, some instructions and system events may causetransactional aborts. Frequent transactional aborts result in wastedcycles and increased inefficiency.

Hardware Lock Elision

Hardware Lock Elision (HLE) provides a legacy compatible instruction setinterface for programmers to use transactional execution. HLE providestwo new instruction prefix hints: XACQUIRE and XRELEASE.

With HLE, a programmer adds the XACQUIRE prefix to the front of theinstruction that is used to acquire the lock that is protecting thecritical section. The processor treats the prefix as a hint to elide thewrite associated with the lock acquire operation. Even though the lockacquire has an associated write operation to the lock, the processordoes not add the address of the lock to the transactional region'swrite-set nor does it issue any write requests to the lock. Instead, theaddress of the lock is added to the read-set. The logical processorenters transactional execution. If the lock was available before theXACQUIRE prefixed instruction, then all other processors will continueto see the lock as available afterwards. Since the transactionallyexecuting logical processor neither added the address of the lock to itswrite-set nor performed externally visible write operations to the lock,other logical processors can read the lock without causing a dataconflict. This allows other logical processors to also enter andconcurrently execute the critical section protected by the lock. Theprocessor automatically detects any data conflicts that occur during thetransactional execution and will perform a transactional abort ifnecessary.

Even though the eliding processor did not perform any external writeoperations to the lock, the hardware ensures program order of operationson the lock. If the eliding processor itself reads the value of the lockin the critical section, it will appear as if the processor had acquiredthe lock, i.e. the read will return the non-elided value. This behaviorallows an HLE execution to be functionally equivalent to an executionwithout the HLE prefixes.

An XRELEASE prefix can be added in front of an instruction that is usedto release the lock protecting a critical section. Releasing the lockinvolves a write to the lock. If the instruction is to restore the valueof the lock to the value the lock had prior to the XACQUIRE prefixedlock acquire operation on the same lock, then the processor elides theexternal write request associated with the release of the lock and doesnot add the address of the lock to the write-set. The processor thenattempts to commit the transactional execution.

With HLE, if multiple threads execute critical sections protected by thesame lock but they do not perform any conflicting operations on eachother's data, then the threads can execute concurrently and withoutserialization. Even though the software uses lock acquisition operationson a common lock, the hardware recognizes this, elides the lock, andexecutes the critical sections on the two threads without requiring anycommunication through the lock—if such communication was dynamicallyunnecessary.

If the processor is unable to execute the region transactionally, thenthe processor will execute the region non-transactionally and withoutelision. HLE enabled software has the same forward progress guaranteesas the underlying non-HLE lock-based execution. For successful HLEexecution, the lock and the critical section code must follow certainguidelines. These guidelines only affect performance, and failure tofollow these guidelines will not result in a functional failure.Hardware without HLE support will ignore the XACQUIRE and XRELEASEprefix hints and will not perform any elision since these prefixescorrespond to the REPNE/REPE IA-32 prefixes which are ignored on theinstructions where XACQUIRE and XRELEASE are valid. Importantly, HLE iscompatible with the existing lock-based programming model. Improper useof hints will not cause functional bugs though it may expose latent bugsalready in the code.

Restricted Transactional Memory (RTM) provides a flexible softwareinterface for transactional execution. RTM provides three newinstructions—XBEGIN, XEND, and XABORT—for programmers to start, commit,and abort a transactional execution.

The programmer uses the XBEGIN instruction to specify the start of atransactional code region and the XEND instruction to specify the end ofthe transactional code region. If the RTM region could not besuccessfully executed transactionally, then the XBEGIN instruction takesan operand that provides a relative offset to the fallback instructionaddress.

A processor may abort RTM transactional execution for many reasons. Inmany instances, the hardware automatically detects transactional abortconditions and restarts execution from the fallback instruction addresswith the architectural state corresponding to that present at the startof the XBEGIN instruction and the EAX register updated to describe theabort status.

The XABORT instruction allows programmers to abort the execution of anRTM region explicitly. The XABORT instruction takes an 8-bit immediateargument that is loaded into the EAX register and will thus be availableto software following an RTM abort. RTM instructions do not have anydata memory location associated with them. While the hardware providesno guarantees as to whether an RTM region will ever successfully committransactionally, most transactions that follow the recommendedguidelines are expected to successfully commit transactionally. However,programmers must always provide an alternative code sequence in thefallback path to guarantee forward progress. This may be as simple asacquiring a lock and executing the specified code regionnon-transactionally. Further, a transaction that always aborts on agiven implementation may complete transactionally on a futureimplementation. Therefore, programmers must ensure the code paths forthe transactional region and the alternative code sequence arefunctionally tested.

Detection of HLE Support

A processor supports HLE execution if CPUID.07H.EBX.HLE [bit 4]=1.However, an application can use the HLE prefixes (XACQUIRE and XRELEASE)without checking whether the processor supports HLE. Processors withoutHLE support ignore these prefixes and will execute the code withoutentering transactional execution.

Detection of RTM Support

A processor supports RTM execution if CPUID.07H.EBX.RTM [bit 11]=1. Anapplication must check if the processor supports RTM before it uses theRTM instructions (XBEGIN, XEND, XABORT). These instructions willgenerate a #UD exception when used on a processor that does not supportRTM.

Detection of XTEST Instruction

A processor supports the XTEST instruction if it supports either HLE orRTM. An application must check either of these feature flags beforeusing the XTEST instruction. This instruction will generate a #UDexception when used on a processor that does not support either HLE orRTM.

Querying Transactional Execution Status

The XTEST instruction can be used to determine the transactional statusof a transactional region specified by HLE or RTM. Note, while the HLEprefixes are ignored on processors that do not support HLE, the XTESTinstruction will generate a #UD exception when used on processors thatdo not support either HLE or RTM.

Requirements for HLE Locks

For HLE execution to successfully commit transactionally, the lock mustsatisfy certain properties and access to the lock must follow certainguidelines.

An XRELEASE prefixed instruction must restore the value of the elidedlock to the value it had before the lock acquisition. This allowshardware to safely elide locks by not adding them to the write-set. Thedata size and data address of the lock release (XRELEASE prefixed)instruction must match that of the lock acquire (XACQUIRE prefixed) andthe lock must not cross a cache line boundary.

Software should not write to the elided lock inside a transactional HLEregion with any instruction other than an XRELEASE prefixed instruction,otherwise such a write may cause a transactional abort. In addition,recursive locks (where a thread acquires the same lock multiple timeswithout first releasing the lock) may also cause a transactional abort.Note that software can observe the result of the elided lock acquireinside the critical section. Such a read operation will return the valueof the write to the lock.

The processor automatically detects violations to these guidelines, andsafely transitions to a non-transactional execution without elision.Since Intel TSX detects conflicts at the granularity of a cache line,writes to data collocated on the same cache line as the elided lock maybe detected as data conflicts by other logical processors eliding thesame lock.

Transactional Nesting

Both HLE and RTM support nested transactional regions. However, atransactional abort restores state to the operation that startedtransactional execution: either the outermost XACQUIRE prefixed HLEeligible instruction or the outermost XBEGIN instruction. The processortreats all nested transactions as one transaction.

HLE Nesting and Elision

Programmers can nest HLE regions up to an implementation specific depthof MAX_HLE_NEST_COUNT. Each logical processor tracks the nesting countinternally but this count is not available to software. An XACQUIREprefixed HLE-eligible instruction increments the nesting count, and anXRELEASE prefixed HLE-eligible instruction decrements it. The logicalprocessor enters transactional execution when the nesting count goesfrom zero to one. The logical processor attempts to commit only when thenesting count becomes zero. A transactional abort may occur if thenesting count exceeds MAX_HLE_NEST_COUNT.

In addition to supporting nested HLE regions, the processor can alsoelide multiple nested locks. The processor tracks a lock for elisionbeginning with the XACQUIRE prefixed HLE eligible instruction for thatlock and ending with the XRELEASE prefixed HLE eligible instruction forthat same lock. The processor can, at any one time, track up to aMAX_HLE_ELIDED_LOCKS number of locks. For example, if the implementationsupports a MAX_HLE_ELIDED_LOCKS value of two and if the programmer neststhree HLE identified critical sections (by performing XACQUIRE prefixedHLE eligible instructions on three distinct locks without performing anintervening XRELEASE prefixed HLE eligible instruction on any one of thelocks), then the first two locks will be elided, but the third won't beelided (but will be added to the transaction's writeset). However, theexecution will still continue transactionally. Once an XRELEASE for oneof the two elided locks is encountered, a subsequent lock acquiredthrough the XACQUIRE prefixed HLE eligible instruction will be elided.

The processor attempts to commit the HLE execution when all elidedXACQUIRE and XRELEASE pairs have been matched, the nesting count goes tozero, and the locks have satisfied requirements. If execution cannotcommit atomically, then execution transitions to a non-transactionalexecution without elision as if the first instruction did not have anXACQUIRE prefix.

RTM Nesting

Programmers can nest RTM regions up to an implementation specificMAX_RTM_NEST_COUNT. The logical processor tracks the nesting countinternally but this count is not available to software. An XBEGINinstruction increments the nesting count, and an XEND instructiondecrements the nesting count. The logical processor attempts to commitonly if the nesting count becomes zero. A transactional abort occurs ifthe nesting count exceeds MAX_RTM_NEST_COUNT.

Nesting HLE and RTM

HLE and RTM provide two alternative software interfaces to a commontransactional execution capability. Transactional processing behavior isimplementation specific when HLE and RTM are nested together, e.g., HLEis inside RTM or RTM is inside HLE-. However, in all cases, theimplementation will maintain HLE and RTM semantics. An implementationmay choose to ignore HLE hints when used inside RTM regions, and maycause a transactional abort when RTM instructions are used inside HLEregions. In the latter case, the transition from transactional tonon-transactional execution occurs seamlessly since the processor willre-execute the HLE region without actually doing elision, and thenexecute the RTM instructions.

Abort Status Definition

RTM uses the EAX register to communicate abort status to software.Following an RTM abort the EAX register has the following definition.

TABLE 1 RTM Abort Status Definition EAX Register Bit Position Meaning 0Set if abort caused by XABORT instruction 1 If set, the transaction maysucceed on retry, this bit is always clear if bit 0 is set 2 Set ifanother logical processor conflicted with a memory address that was partof the transaction that aborted 3 Set if an internal buffer overflowed 4Set if a debug breakpoint was hit 5 Set if an abort occurred duringexecution of a nested transaction 23:6 Reserved 31-24 XABORT argument(only valid if bit 0 set, otherwise reserved)

The EAX abort status for RTM only provides causes for aborts. It doesnot by itself encode whether an abort or commit occurred for the RTMregion. The value of EAX can be 0 following an RTM abort. For example, aCPUID instruction when used inside an RTM region causes a transactionalabort and may not satisfy the requirements for setting any of the EAXbits. This may result in an EAX value of 0.

RTM Memory Ordering

A successful RTM commit causes all memory operations in the RTM regionto appear to execute atomically. A successfully committed RTM regionconsisting of an XBEGIN followed by an XEND, even with no memoryoperations in the RTM region, has the same ordering semantics as a LOCKprefixed instruction.

The XBEGIN instruction does not have fencing semantics. However, if anRTM execution aborts, then all memory updates from within the RTM regionare discarded and are not made visible to any other logical processor.

RTM-Enabled Debugger Support

By default, any debug exception inside an RTM region will cause atransactional abort and will redirect control flow to the fallbackinstruction address with architectural state recovered and bit 4 in EAXset. However, to allow software debuggers to intercept execution ondebug exceptions, the RTM architecture provides additional capability.

If bit 11 of DR7 and bit 15 of the IA32_DEBUGCTL_MSR are both 1, any RTMabort due to a debug exception (#DB) or breakpoint exception (#BP)causes execution to roll back and restart from the XBEGIN instructioninstead of the fallback address. In this scenario, the EAX register willalso be restored back to the point of the XBEGIN instruction.

Programming Considerations

Typical programmer-identified regions are expected to transactionallyexecute and commit successfully. However, Intel TSX does not provide anysuch guarantee. A transactional execution may abort for many reasons. Totake full advantage of the transactional capabilities, programmersshould follow certain guidelines to increase the probability of theirtransactional execution committing successfully.

This section discusses various events that may cause transactionalaborts. The architecture ensures that updates performed within atransaction that subsequently aborts execution will never becomevisible. Only committed transactional executions initiate an update tothe architectural state. Transactional aborts never cause functionalfailures and only affect performance.

Instruction Based Considerations

Programmers can use any instruction safely inside a transaction (HLE orRTM) and can use transactions at any privilege level. However, someinstructions will always abort the transactional execution and causeexecution to seamlessly and safely transition to a non-transactionalpath.

Intel TSX allows for most common instructions to be used insidetransactions without causing aborts. The following operations inside atransaction do not typically cause an abort:

-   -   Operations on the instruction pointer register, general purpose        registers (GPRs) and the status flags (CF, OF, SF, PF, AF, and        ZF); and    -   Operations on XMM and YMM registers and the MXCSR register.

However, programmers must be careful when intermixing SSE and AVXoperations inside a transactional region. Intermixing SSE instructionsaccessing XMM registers and AVX instructions accessing YMM registers maycause transactions to abort. Programmers may use REP/REPNE prefixedstring operations inside transactions. However, long strings may causeaborts. Further, the use of CLD and STD instructions may cause aborts ifthey change the value of the DF flag. However, if DF is 1, the STDinstruction will not cause an abort. Similarly, if DF is 0, then the CLDinstruction will not cause an abort.

Instructions not enumerated here as causing abort when used inside atransaction will typically not cause a transaction to abort (examplesinclude but are not limited to MFENCE, LFENCE, SFENCE, RDTSC, RDTSCP,etc.).

The following instructions will abort transactional execution on anyimplementation:

-   -   XABORT    -   CPUID    -   PAUSE

In addition, in some implementations, the following instructions mayalways cause transactional aborts. These instructions are not expectedto be commonly used inside typical transactional regions. However,programmers must not rely on these instructions to force a transactionalabort, since whether they cause transactional aborts is implementationdependent.

-   -   Operations on X87 and MMX architecture state. This includes all        MMX and X87 instructions, including the FXRSTOR and FXSAVE        instructions.    -   Update to non-status portion of EFLAGS: CLI, STI, POPFD, POPFQ,        CLTS.    -   Instructions that update segment registers, debug registers        and/or control registers: MOV to DS/ES/FS/GS/SS, POP        DS/ES/FS/GS/SS, LDS, LES, LFS, LGS, LSS, SWAPGS, WRFSBASE,        WRGSBASE, LGDT, SGDT, LIDT, SIDT, LLDT, SLDT, LTR, STR, Far        CALL, Far JMP, Far RET, IRET, MOV to DRx, MOV to        CR0/CR2/CR3/CR4/CR8 and LMSW.    -   Ring transitions: SYSENTER, SYSCALL, SYSEXIT, and SYSRET.    -   TLB and Cacheability control: CLFLUSH, INVD, WBINVD, INVLPG,        INVPCID, and memory instructions with a non-temporal hint        (MOVNTDQA, MOVNTDQ, MOVNTI, MOVNTPD, MOVNTPS, and MOVNTQ).    -   Processor state save: XSAVE, XSAVEOPT, and XRSTOR.    -   Interrupts: INTn, INTO.    -   IO: IN, INS, REP INS, OUT, OUTS, REP OUTS and their variants.    -   VMX: VMPTRLD, VMPTRST, VMCLEAR, VMREAD, VMWRITE, VMCALL,        VMLAUNCH, VMRESUME, VMXOFF, VMXON, INVEPT, and INVVPID.    -   SMX: GETSEC.    -   UD2, RSM, RDMSR, WRMSR, HLT, MONITOR, MWAIT, XSETBV, VZEROUPPER,        MASKMOVQ, and V/MASKMOVDQU.

Runtime Considerations

In addition to the instruction-based considerations, runtime events maycause transactional execution to abort. These may be due to data accesspatterns or micro-architectural implementation features. The followinglist is not a comprehensive discussion of all abort causes.

Any fault or trap in a transaction that must be exposed to software willbe suppressed. Transactional execution will abort and execution willtransition to a non-transactional execution, as if the fault or trap hadnever occurred. If an exception is not masked, then that un-maskedexception will result in a transactional abort and the state will appearas if the exception had never occurred.

Synchronous exception events (#DE, #OF, #NP, #SS, #GP, #BR, #UD, #AC,#XF, #PF, #NM, #TS, #MF, #DB, #BP/INT3) that occur during transactionalexecution may cause an execution not to commit transactionally, andrequire a non-transactional execution. These events are suppressed as ifthey had never occurred. With HLE, since the non-transactional code pathis identical to the transactional code path, these events will typicallyre-appear when the instruction that caused the exception is re-executednon-transactionally, causing the associated synchronous events to bedelivered appropriately in the non-transactional execution. Asynchronousevents (NMI, SMI, INTR, IPI, PMI, etc.) occurring during transactionalexecution may cause the transactional execution to abort and transitionto a non-transactional execution. The asynchronous events will be pendedand handled after the transactional abort is processed.

Transactions only support write-back cacheable memory type operations. Atransaction may always abort if the transaction includes operations onany other memory type. This includes instruction fetches to UC memorytype.

Memory accesses within a transactional region may require the processorto set the Accessed and Dirty flags of the referenced page table entry.The behavior of how the processor handles this is implementationspecific. Some implementations may allow the updates to these flags tobecome externally visible even if the transactional region subsequentlyaborts. Some Intel TSX implementations may choose to abort thetransactional execution if these flags need to be updated. Further, aprocessor's page-table walk may generate accesses to its owntransactionally written but uncommitted state. Some Intel TSXimplementations may choose to abort the execution of a transactionalregion in such situations. Regardless, the architecture ensures that, ifthe transactional region aborts, then the transactionally written statewill not be made architecturally visible through the behavior ofstructures such as TLBs.

Executing self-modifying code transactionally may also causetransactional aborts. Programmers must continue to follow the Intelrecommended guidelines for writing self-modifying and cross-modifyingcode even when employing HLE and RTM. While an implementation of RTM andHLE will typically provide sufficient resources for executing commontransactional regions, implementation constraints and excessive sizesfor transactional regions may cause a transactional execution to abortand transition to a non-transactional execution. The architectureprovides no guarantee of the amount of resources available to dotransactional execution and does not guarantee that a transactionalexecution will ever succeed.

Conflicting requests to a cache line accessed within a transactionalregion may prevent the transaction from executing successfully. Forexample, if logical processor P0 reads line A in a transactional regionand another logical processor P1 writes line A (either inside or outsidea transactional region) then logical processor P0 may abort if logicalprocessor P1's write interferes with processor P0's ability to executetransactionally.

Similarly, if P0 writes line A in a transactional region and P1 reads orwrites line A (either inside or outside a transactional region), then P0may abort if P1's access to line A interferes with P0's ability toexecute transactionally. In addition, other coherence traffic may attimes appear as conflicting requests and may cause aborts. While thesefalse conflicts may happen, they are expected to be uncommon. Theconflict resolution policy to determine whether P0 or P1 aborts in theabove scenarios is implementation specific.

Generic Transaction Execution Embodiments:

According to “ARCHITECTURES FOR TRANSACTIONAL MEMORY”, a dissertationsubmitted to the Department of Computer Science and the Committee onGraduate Studies of Stanford University in partial fulfillment of therequirements for the Degree of Doctor of Philosophy, by Austen McDonald,June 2009, incorporated by reference herein in its entirety,fundamentally, there are three mechanisms needed to implement an atomicand isolated transactional region: versioning, conflict detection, andcontention management.

To make a transactional code region appear atomic, all the modificationsperformed by that transactional code region must be stored and keptisolated from other transactions until commit time. The system does thisby implementing a versioning policy. Two versioning paradigms exist:eager and lazy. An eager versioning system stores newly generatedtransactional values in-place and stores previous memory values on theside, in what is called an undo-log. A lazy versioning system stores newvalues temporarily in what is called a write buffer, copying them tomemory only on commit. In either system, the cache is used to optimizestorage of new versions.

To ensure that transactions appear to be performed atomically, conflictsmust be detected and resolved. The two systems, i.e., the eager and lazyversioning systems, detect conflicts by implementing a conflictdetection policy, either optimistic or pessimistic. An optimistic systemexecutes transactions in parallel, checking for conflicts only when atransaction commits. A pessimistic systems check for conflicts at eachload and store. Similar to versioning, conflict detection also uses thecache, marking each line as either part of the read-set, part of thewrite-set, or both. The two systems resolve conflicts by implementing acontention management policy. Many contention management policies exist,some are more appropriate for optimistic conflict detection and some aremore appropriate for pessimistic. Described below are some examplepolicies.

Since each transactional memory (TM) system needs both versioningdetection and conflict detection, these options give rise to fourdistinct TM designs: Eager-Pessimistic (EP), Eager-Optimistic (EO),Lazy-Pessimistic (LP), and Lazy-Optimistic (LO). Table 2 brieflydescribes all four distinct TM designs.

FIGS. 1 and 2 depict an example of a multicore TM environment. FIG. 1shows many TM-enabled CPUs (CPU1, CPU2, etc.) on one die, connected withan interconnect. Each CPU (also known as a Processor) may have a splitcache consisting of an Instruction Cache for caching instructions frommemory to be executed and a Data Cache with TM support for caching data(operands) of memory locations to be operated on by the CPU. In animplementation, caches of multiple dies are interconnected to supportcache coherency between the caches of the multiple dies. In animplementation, a single cache, rather than the split cache is employedholding both instructions and data. In implementations, the CPU cachesare one level of caching in a hierarchical cache structure. For exampleeach die may employ a shared cache to be shared amongst all the CPUs onthe die. In another implementation, each die may have access to a sharedcache, shared amongst all the processors of all the dies.

FIG. 2 shows the details of an example transactional CPU, includingadditions to support TM. The transactional CPU (processor) may includehardware for supporting Register Checkpoints and special TM Registers.The transactional CPU cache may have the MESI bits, Tags and Data of aconventional cache but also, for example, R bits showing a line has beenread by the CPU while executing a transaction and W bits showing a linehas been written-to by the CPU while executing a transaction.

A key detail for programmers in any TM system is how non-transactionalaccesses interact with transactions. By design, transactional accessesare screened from each other using the mechanisms above. However, theinteraction between a regular, non-transactional load with a transactioncontaining a new value for that address must still be considered. Inaddition, the interaction between a non-transactional store with atransaction that has read that address must also be explored. These areissues of the database concept isolation.

A TM system is said to implement strong isolation, sometimes calledstrong atomicity, when every non-transactional load and store acts likean atomic transaction. Therefore, non-transactional loads cannot seeuncommitted data and non-transactional stores cause atomicity violationsin any transactions that have read that address. A system where this isnot the case is said to implement weak isolation, sometimes called weakatomicity.

Strong isolation is often more desirable than weak isolation due to therelative ease of conceptualization and implementation of strongisolation. Additionally, if a programmer has forgotten to surround someshared memory references with transactions, causing bugs, then withstrong isolation, the programmer will often detect that oversight usinga simple debug interface because the programmer will see anon-transactional region causing atomicity violations. Also, programswritten in one model may work differently on another model.

Further, strong isolation is often easier to support in hardware TM thanweak isolation. With strong isolation, since the coherence protocolalready manages load and store communication between processors,transactions can detect non-transactional loads and stores and actappropriately. To implement strong isolation in software TransactionalMemory (TM), non-transactional code must be modified to include read-and write-barriers; potentially crippling performance. Although greateffort has been expended to remove many un-needed barriers, suchtechniques are often complex and performance is typically far lower thanthat of HTMs.

TABLE 2 Transactional Memory Design Space VERSIONING Lazy Eager CONFLICTOptimistic Storing updates in a write buffer; Not practical: waiting toupdate memory DETECTION detecting conflicts at commit time. until committime but detecting conflicts at access time guarantees wasted work andprovides no advantage Pessimistic Storing updates in a write buffer;Updating memory, keeping old values in detecting conflicts at accesstime. undo log; detecting conflicts at access time.

Table 2 illustrates the fundamental design space of transactional memory(versioning and conflict detection).

Eager-Pessimistic (EP)

This first TM design described below is known as Eager-Pessimistic. AnEP system stores its write-set “in-place” (hence the name “eager”) and,to support rollback, stores the old values of overwritten lines in an“undo log”. Processors use the W and R cache bits to track read andwrite-sets and detect conflicts when receiving snooped load requests.Perhaps the most notable examples of EP systems in known literature areLogTM and UTM.

Beginning a transaction in an EP system is much like beginning atransaction in other systems: tm_begin( ) takes a register checkpoint,and initializes any status registers. An EP system also requiresinitializing the undo log, the details of which are dependent on the logformat, but often involve initializing a log base pointer to a region ofpre-allocated, thread-private memory, and clearing a log boundsregister.

Versioning: In EP, due to the way eager versioning is designed tofunction, the MESI state transitions (cache line indicatorscorresponding to Modified, Exclusive, Shared, and Invalid code states)are left mostly unchanged. Outside of a transaction, the MESI statetransitions are left completely unchanged. When reading a line inside atransaction, the standard coherence transitions apply (S (Shared)→S, I(Invalid)→S, or I→E (Exclusive)), issuing a load miss as needed, but theR bit is also set. Likewise, writing a line applies the standardtransitions (S→M, E→I, I→M), issuing a miss as needed, but also sets theW (Written) bit. The first time a line is written, the old version ofthe entire line is loaded then written to the undo log to preserve it incase the current transaction aborts. The newly written data is thenstored “in-place,” over the old data.

Conflict Detection: Pessimistic conflict detection uses coherencemessages exchanged on misses, or upgrades, to look for conflicts betweentransactions. When a read miss occurs within a transaction, otherprocessors receive a load request; but they ignore the request if theydo not have the needed line. If the other processors have the neededline non-speculatively or have the line R (Read), they downgrade thatline to S, and in certain cases issue a cache-to-cache transfer if theyhave the line in MESI's M or E state. However, if the cache has the lineW, then a conflict is detected between the two transactions andadditional action(s) must be taken.

Similarly, when a transaction seeks to upgrade a line from shared tomodified (on a first write), the transaction issues an exclusive loadrequest, which is also used to detect conflicts. If a receiving cachehas the line non-speculatively, then the line is invalidated, and incertain cases a cache-to-cache transfer (M or E states) is issued. But,if the line is R or W, a conflict is detected.

Validation: Because conflict detection is performed on every load, atransaction always has exclusive access to its own write-set. Therefore,validation does not require any additional work.

Commit: Since eager versioning stores the new version of data itemsin-place, the commit process simply clears the W and R bits and discardsthe undo log.

Abort: When a transaction rolls back, the original version of each cacheline in the undo log must be restored, a process called “unrolling” or“applying” the log. This is done during tm_discard( ) and must be atomicwith regard to other transactions. Specifically, the write-set muststill be used to detect conflicts: this transaction has the only correctversion of lines in its undo log, and requesting transactions must waitfor the correct version to be restored from that log. Such a log can beapplied using a hardware state machine or software abort handler.

Eager-Pessimistic has the characteristics of: Commit is simple and sinceit is in-place, very fast. Similarly, validation is a no-op. Pessimisticconflict detection detects conflicts early, thereby reducing the numberof “doomed” transactions. For example, if two transactions are involvedin a Write-After-Read dependency, then that dependency is detectedimmediately in pessimistic conflict detection. However, in optimisticconflict detection such conflicts are not detected until the writercommits.

Eager-Pessimistic also has the characteristics of: As described above,the first time a cache line is written, the old value must be written tothe log, incurring extra cache accesses. Aborts are expensive as theyrequire undoing the log. For each cache line in the log, a load must beissued, perhaps going as far as main memory before continuing to thenext line. Pessimistic conflict detection also prevents certainserializable schedules from existing.

Additionally, because conflicts are handled as they occur, there is apotential for livelock and careful contention management mechanisms mustbe employed to guarantee forward progress.

Lazy-Optimistic (LO)

Another popular TM design is Lazy-Optimistic (LO), which stores itswrite-set in a “write buffer” or “redo log” and detects conflicts atcommit time (still using the R and W bits).

Versioning: Just as in the EP system, the MESI protocol of the LO designis enforced outside of the transactions. Once inside a transaction,reading a line incurs the standard MESI transitions but also sets the Rbit. Likewise, writing a line sets the W bit of the line, but handlingthe MESI transitions of the LO design is different from that of the EPdesign. First, with lazy versioning, the new versions of written dataare stored in the cache hierarchy until commit while other transactionshave access to old versions available in memory or other caches. To makeavailable the old versions, dirty lines (M lines) must be evicted whenfirst written by a transaction. Second, no upgrade misses are neededbecause of the optimistic conflict detection feature: if a transactionhas a line in the S state, it can simply write to it and upgrade thatline to an M state without communicating the changes with othertransactions because conflict detection is done at commit time.

Conflict Detection and Validation: To validate a transaction and detectconflicts, LO communicates the addresses of speculatively modified linesto other transactions only when it is preparing to commit. Onvalidation, the processor sends one, potentially large, network packetcontaining all the addresses in the write-set. Data is not sent, butleft in the cache of the committer and marked dirty (M). To build thispacket without searching the cache for lines marked W, a simple bitvector is used, called a “store buffer,” with one bit per cache line totrack these speculatively modified lines. Other transactions use thisaddress packet to detect conflicts: if an address is found in the cacheand the R and/or W bits are set, then a conflict is initiated. If theline is found but neither R nor W is set, then the line is simplyinvalidated, which is similar to processing an exclusive load.

To support transaction atomicity, these address packets must be handledatomically, i.e., no two address packets may exist at once with the sameaddresses. In an LO system, this can be achieved by simply acquiring aglobal commit token before sending the address packet. However, atwo-phase commit scheme could be employed by first sending out theaddress packet, collecting responses, enforcing an ordering protocol(perhaps oldest transaction first), and committing once all responsesare satisfactory.

Commit: Once validation has occurred, commit needs no special treatment:simply clear W and R bits and the store buffer. The transaction's writesare already marked dirty in the cache and other caches' copies of theselines have been invalidated via the address packet. Other processors canthen access the committed data through the regular coherence protocol.

Abort: Rollback is equally easy: because the write-set is containedwithin the local caches, these lines can be invalidated, then clear Wand R bits and the store buffer. The store buffer allows W lines to befound to invalidate without the need to search the cache.

Lazy-Optimistic has the characteristics of: Aborts are very fast,requiring no additional loads or stores and making only local changes.More serializable schedules can exist than found in EP, which allows anLO system to more aggressively speculate that transactions areindependent, which can yield higher performance. Finally, the latedetection of conflicts can increase the likelihood of forward progress.

Lazy-Optimistic also has the characteristics of: Validation takes globalcommunication time proportional to size of write set. Doomedtransactions can waste work since conflicts are detected only at committime.

Lazy-Pessimistic (LP)

Lazy-Pessimistic (LP) represents a third TM design option, sittingsomewhere between EP and LO: storing newly written lines in a writebuffer but detecting conflicts on a per access basis.

Versioning: Versioning is similar but not identical to that of LO:reading a line sets its R bit, writing a line sets its W bit, and astore buffer is used to track W lines in the cache. Also, dirty (M)lines must be evicted when first written by a transaction, just as inLO. However, since conflict detection is pessimistic, load exclusivesmust be performed when upgrading a transactional line from I, S→M, whichis unlike LO.

Conflict Detection: LP's conflict detection operates the same as EP's:using coherence messages to look for conflicts between transactions.

Validation: Like in EP, pessimistic conflict detection ensures that atany point, a running transaction has no conflicts with any other runningtransaction, so validation is a no-op.

Commit: Commit needs no special treatment: simply clear W and R bits andthe store buffer, like in LO.

Abort: Rollback is also like that of LO: simply invalidate the write-setusing the store buffer and clear the W and R bits and the store buffer.

Eager-Optimistic (EO)

The LP has the characteristics of: Like LO, aborts are very fast. LikeEP, the use of pessimistic conflict detection reduces the number of“doomed” transactions Like EP, some serializable schedules are notallowed and conflict detection must be performed on each cache miss.

The final combination of versioning and conflict detection isEager-Optimistic (EO). EO may be a less than optimal choice for HTMsystems: since new transactional versions are written in-place, othertransactions have no choice but to notice conflicts as they occur (i.e.,as cache misses occur). But since EO waits until commit time to detectconflicts, those transactions become “zombies,” continuing to execute,wasting resources, yet are “doomed” to abort.

EO has proven to be useful in STMs and is implemented by Bartok-STM andMcRT. A lazy versioning STM needs to check its write buffer on each readto ensure that it is reading the most recent value. Since the writebuffer is not a hardware structure, this is expensive, hence thepreference for write-in-place eager versioning. Additionally, sincechecking for conflicts is also expensive in an STM, optimistic conflictdetection offers the advantage of performing this operation in bulk.

Contention Management

How a transaction rolls back once the system has decided to abort thattransaction has been described above, but, since a conflict involves twotransactions, the topics of which transaction should abort, how thatabort should be initiated, and when should the aborted transaction beretried need to be explored. These are topics that are addressed byContention Management (CM), a key component of transactional memory.Described below are policies regarding how the systems initiate abortsand the various established methods of managing which transactionsshould abort in a conflict.

Contention Management Policies

A Contention Management (CM) Policy is a mechanism that determines whichtransaction involved in a conflict should abort and when the abortedtransaction should be retried. For example, it is often the case thatretrying an aborted transaction immediately does not lead to the bestperformance. Conversely, employing a back-off mechanism, which delaysthe retrying of an aborted transaction, can yield better performance.STMs first grappled with finding the best contention management policiesand many of the policies outlined below were originally developed forSTMs.

CM Policies draw on a number of measures to make decisions, includingages of the transactions, size of read- and write-sets, the number ofprevious aborts, etc. The combinations of measures to make suchdecisions are endless, but certain combinations are described below,roughly in order of increasing complexity.

To establish some nomenclature, first note that in a conflict there aretwo sides: the attacker and the defender. The attacker is thetransaction requesting access to a shared memory location. Inpessimistic conflict detection, the attacker is the transaction issuingthe load or load exclusive. In optimistic, the attacker is thetransaction attempting to validate. The defender in both cases is thetransaction receiving the attacker's request.

An Aggressive CM Policy immediately and always retries either theattacker or the defender. In LO, Aggressive means that the attackeralways wins, and so Aggressive is sometimes called committer wins. Sucha policy was used for the earliest LO systems. In the case of EP,Aggressive can be either defender wins or attacker wins.

Restarting a conflicting transaction that will immediately experienceanother conflict is bound to waste work—namely interconnect bandwidthrefilling cache misses. A Polite CM Policy employs exponential backoff(but linear could also be used) before restarting conflicts. To preventstarvation, a situation where a process does not have resourcesallocated to it by the scheduler, the exponential backoff greatlyincreases the odds of transaction success after some n retries.

Another approach to conflict resolution is to randomly abort theattacker or defender (a policy called Randomized). Such a policy may becombined with a randomized backoff scheme to avoid unneeded contention.

However, making random choices, when selecting a transaction to abort,can result in aborting transactions that have completed “a lot of work”,which can waste resources. To avoid such waste, the amount of workcompleted on the transaction can be taken into account when determiningwhich transaction to abort. One measure of work could be a transaction'sage. Other methods include Oldest, Bulk TM, Size Matters, Karma, andPolka. Oldest is a simple timestamp method that aborts the youngertransaction in a conflict. Bulk TM uses this scheme. Size Matters islike Oldest but instead of transaction age, the number of read/writtenwords is used as the priority, reverting to Oldest after a fixed numberof aborts. Karma is similar, using the size of the write-set aspriority. Rollback then proceeds after backing off a fixed amount oftime. Aborted transactions keep their priorities after being aborted(hence the name Karma). Polka works like Karma but instead of backingoff a predefined amount of time, it backs off exponentially more eachtime.

Since aborting wastes work, it is logical to argue that stalling anattacker until the defender has finished their transaction would lead tobetter performance. Unfortunately, such a simple scheme easily leads todeadlock.

Deadlock avoidance techniques can be used to solve this problem. Greedyuses two rules to avoid deadlock. The first rule is, if a firsttransaction, T1, has lower priority than a second transaction, T0, or ifT1 is waiting for another transaction, then T1 aborts when conflictingwith T0. The second rule is, if T1 has higher priority than T0 and isnot waiting, then T0 waits until T1 commits, aborts, or starts waiting(in which case the first rule is applied). Greedy provides someguarantees about time bounds for executing a set of transactions. One EPdesign (LogTM) uses a CM policy similar to Greedy to achieve stallingwith conservative deadlock avoidance.

Example MESI coherency rules provide for four possible states in which acache line of a multiprocessor cache system may reside, M, E, S, and I,defined as follows:

Modified (M): The cache line is present only in the current cache, andis dirty; it has been modified from the value in main memory. The cacheis required to write the data back to main memory at some time in thefuture, before permitting any other read of the (no longer valid) mainmemory state. The write-back changes the line to the Exclusive state.

Exclusive (E): The cache line is present only in the current cache, butis clean; it matches main memory. It may be changed to the Shared stateat any time, in response to a read request. Alternatively, it may bechanged to the Modified state when writing to it.

Shared (S): Indicates that this cache line may be stored in other cachesof the machine and is “clean”; it matches the main memory. The line maybe discarded (changed to the Invalid state) at any time.

Invalid (I): Indicates that this cache line is invalid (unused).

TM coherency status indicators (R, W) may be provided for each cacheline, in addition to, or encoded in the MESI coherency bits. An Rindicator indicates the current transaction has read from the data ofthe cache line, and a W indicator indicates the current transaction haswritten to the data of the cache line.

In another aspect of TM design, a system is designed using transactionalstore buffers. U.S. Pat. No. 6,349,361 titled “Methods and Apparatus forReordering and Renaming Memory References in a Multiprocessor ComputerSystem,” filed Mar. 31, 2000 and incorporated by reference herein in itsentirety, teaches a method for reordering and renaming memory referencesin a multiprocessor computer system having at least a first and a secondprocessor. The first processor has a first private cache and a firstbuffer, and the second processor has a second private cache and a secondbuffer. The method includes the steps of, for each of a plurality ofgated store requests received by the first processor to store a datum,exclusively acquiring a cache line that contains the datum by the firstprivate cache, and storing the datum in the first buffer. Upon the firstbuffer receiving a load request from the first processor to load aparticular datum, the particular datum is provided to the firstprocessor from among the data stored in the first buffer based on anin-order sequence of load and store operations. Upon the first cachereceiving a load request from the second cache for a given datum, anerror condition is indicated and a current state of at least one of theprocessors is reset to an earlier state when the load request for thegiven datum corresponds to the data stored in the first buffer.

The main implementation components of one such transactional memoryfacility are a transaction-backup register file for holdingpre-transaction GR (general register) content, a cache directory totrack the cache lines accessed during the transaction, a store cache tobuffer stores until the transaction ends, and firmware routines toperform various complex functions. In this section a detailedimplementation is described.

IBM zEnterprise EC12 Enterprise Server Embodiment

The IBM zEnterprise EC12 enterprise server introduces transactionalexecution (TX) in transactional memory, and is described in part in apaper, “Transactional Memory Architecture and Implementation for IBMSystem z” of Proceedings Pages 25-36 presented at MICRO-45, 1-5 Dec.2012, Vancouver, British Columbia, Canada, available from IEEE ComputerSociety Conference Publishing Services (CPS), which is incorporated byreference herein in its entirety.

Table 3 shows an example transaction. Transactions started with TBEGINare not assured to ever successfully complete with TEND, since they canexperience an aborting condition at every attempted execution, e.g., dueto repeating conflicts with other CPUs. This requires that the programsupports a fallback path to perform the same operationnon-transactionally, e.g., by using traditional locking schemes. Thisputs significant burden on the programming and software verificationteams, especially where the fallback path is not automatically generatedby a reliable compiler.

TABLE 3 Example Transaction Code LHI R0,0 *initialize retry count=0 loopTBEGIN *begin transaction JNZ abort *go to abort code if CC1=0 LT R1,lock *load and test the fallback lock JNZ lckbzy *branch if lock busy .. . perform operation . . . TEND *end transaction . . . . . . . . . . .. lckbzy TABORT *abort if lock busy; this *resumes after TBEGIN abort JOfallback *no retry if CC=3 AHI R0, 1 *increment retry count CIJNL R0,6,fallback *give up after 6 attempts PPA R0, TX *random delay based onretry count . . . potentially wait for lock to become free . . . J loop*jump back to retryfallback OBTAIN lock *using Compare&Swap . . .perform operation . . . RELEASE lock . . . . . . . . . . . .

The requirement of providing a fallback path for aborted TransactionExecution (TX) transactions can be onerous. Many transactions operatingon shared data structures are expected to be short, touch only a fewdistinct memory locations, and use simple instructions only. For thosetransactions, the IBM zEnterprise EC12 introduces the concept ofconstrained transactions; under normal conditions, the CPU assures thatconstrained transactions eventually end successfully, albeit withoutgiving a strict limit on the number of necessary retries. A constrainedtransaction starts with a TBEGINC instruction and ends with a regularTEND. Implementing a task as a constrained or non-constrainedtransaction typically results in very comparable performance, butconstrained transactions simplify software development by removing theneed for a fallback path. IBM's Transactional Execution architecture isfurther described in z/Architecture, Principles of Operation, TenthEdition, SA22-7832-09 published September 2012 from IBM, incorporated byreference herein in its entirety.

A constrained transaction starts with the TBEGINC instruction. Atransaction initiated with TBEGINC must follow a list of programmingconstraints; otherwise the program takes a non-filterableconstraint-violation interruption. Exemplary constraints may include,but not be limited to: the transaction can execute a maximum of 32instructions, all instruction text must be within 256 consecutive bytesof memory; the transaction contains only forward-pointing relativebranches (i.e., no loops or subroutine calls); the transaction canaccess a maximum of 4 aligned octowords (an octoword is 32 bytes) ofmemory; and restriction of the instruction-set to exclude complexinstructions like decimal or floating-point operations. The constraintsare chosen such that many common operations like doubly linkedlist-insert/delete operations can be performed, including the verypowerful concept of atomic compare-and-swap targeting up to 4 alignedoctowords. At the same time, the constraints were chosen conservativelysuch that future CPU implementations can assure transaction successwithout needing to adjust the constraints, since that would otherwiselead to software incompatibility.

TBEGINC mostly behaves like XBEGIN in TSX or TBEGIN on IBM's zEC12servers, except that the floating-point register (FPR) control and theprogram interruption filtering fields do not exist and the controls areconsidered to be zero. On a transaction abort, the instruction addressis set back directly to the TBEGINC instead of to the instruction after,reflecting the immediate retry and absence of an abort path forconstrained transactions.

Nested transactions are not allowed within constrained transactions, butif a TBEGINC occurs within a non-constrained transaction it is treatedas opening a new non-constrained nesting level just like TBEGIN would.This can occur, e.g., if a non-constrained transaction calls asubroutine that uses a constrained transaction internally.

Since interruption filtering is implicitly off, all exceptions during aconstrained transaction lead to an interruption into the operatingsystem (OS). Eventual successful finishing of the transaction relies onthe capability of the OS to page-in the at most 4 pages touched by anyconstrained transaction. The OS must also ensure time-slices long enoughto allow the transaction to complete.

TABLE 4 Transaction Code Example TBEGINC *begin constrained transaction. . . perform operation . . . TEND *end transaction

Table 4 shows the constrained-transactional implementation of the codein Table 3, assuming that the constrained transactions do not interactwith other locking-based code. No lock testing is shown therefore, butcould be added if constrained transactions and lock-based code weremixed.

When failure occurs repeatedly, software emulation is performed usingmillicode as part of system firmware. Advantageously, constrainedtransactions have desirable properties because of the burden removedfrom programmers.

The IBM zEnterprise EC12 processor introduced the transactionalexecution facility. The processor can decode 3 instructions per clockcycle; simple instructions are dispatched as single micro-ops, and morecomplex instructions are cracked into multiple micro-ops. The micro-ops(Uops) are written into a unified issue queue, from where they can beissued out-of-order. Up to two fixed-point, one floating-point, twoload/store, and two branch instructions can execute every cycle. AGlobal Completion Table (GCT) holds every micro-op. The GCT is writtenin-order at decode time, tracks the execution status of each micro-op,and completes instructions when all micro-ops of the oldest instructiongroup have successfully executed.

The level 1 (L1) data cache is a 96 KB (kilo-byte) 6-way associativecache with 256 byte cache-lines and 4 cycle use latency, coupled to aprivate 1 MB (mega-byte) 8-way associative 2nd-level (L2) data cachewith 7 cycles use-latency penalty for L1 misses. L1 cache is the cacheclosest to a processor and Ln cache is a cache at the nth level ofcaching. Both L1 and L2 caches are store-through. Six cores on eachcentral processor (CP) chip share a 48 MB 3rd-level store-in cache, andsix CP chips are connected to an off-chip 384 MB 4th-level cache,packaged together on a glass ceramic multi-chip module (MCM). Up to 4multi-chip modules (MCMs) can be connected to a coherent symmetricmulti-processor (SMP) system with up to 144 cores (not all cores areavailable to run customer workload).

Coherency is managed with a variant of the MESI protocol. Cache-linescan be owned read-only (shared) or exclusive; the L1 and L2 arestore-through and thus do not contain dirty lines. The L3 and L4 cachesare store-in and track dirty states. Each cache is inclusive of all itsconnected lower level caches.

Coherency requests are called “cross interrogates” (XI) and are senthierarchically from higher level to lower-level caches, and between theL4s. When one core misses the L1 and L2 and requests the cache line fromits local L3, the L3 checks whether it owns the line, and if necessarysends an XI to the currently owning L2/L1 under that L3 to ensurecoherency, before it returns the cache line to the requestor. If therequest also misses the L3, the L3 sends a request to the L4 whichenforces coherency by sending XIs to all necessary L3s under that L4,and to the neighboring L4s. Then the L4 responds to the requesting L3which forwards the response to the L2/L1.

Note that due to the inclusivity rule of the cache hierarchy, sometimescache lines are XI'ed from lower-level caches due to evictions onhigher-level caches caused by associativity overflows from requests toother cache lines. These XIs can be called “LRU XIs”, where LRU standsfor least recently used.

Making reference to yet another type of XI requests, Demote-XIstransition cache-ownership from exclusive into read-only state, andExclusive-XIs transition cache ownership from exclusive into invalidstate. Demote-XIs and Exclusive-XIs need a response back to the XIsender. The target cache can “accept” the XI, or send a “reject”response if it first needs to evict dirty data before accepting the XI.The L1/L2 caches are store through, but may reject demote-XIs andexclusive XIs if they have stores in their store queues that need to besent to L3 before downgrading the exclusive state. A rejected XI will berepeated by the sender. Read-only-XIs are sent to caches that own theline read-only; no response is needed for such XIs since they cannot berejected. The details of the SMP protocol are similar to those describedfor the IBM z10 by P. Mak, C. Walters, and G. Strait, in “IBM System z10processor cache subsystem microarchitecture”, IBM Journal of Researchand Development, Vol 53:1, 2009, which is incorporated by referenceherein in its entirety.

Transactional Instruction Execution

FIG. 2 depicts example components of an example CPU. The instructiondecode unit (IDU) keeps track of the current transaction nesting depth(TND). When the IDU receives a TBEGIN instruction, the nesting depth isincremented, and conversely decremented on TEND instructions. Thenesting depth is written into the GCT for every dispatched instruction.When a TBEGIN or TEND is decoded on a speculative path that later getsflushed, the IDU's nesting depth is refreshed from the youngest GCTentry that is not flushed. The transactional state is also written intothe issue queue for consumption by the execution units, mostly by theLoad/Store Unit (LSU). The TBEGIN instruction may specify a transactiondiagnostic block (TDB) for recording status information, should thetransaction abort before reaching a TEND instruction.

Similar to the nesting depth, the IDU/GCT collaboratively track theaccess register/floating-point register (AR/FPR) modification masksthrough the transaction nest; the IDU can place an abort request intothe GCT when an AR/FPR-modifying instruction is decoded and themodification mask blocks that. When the instruction becomesnext-to-complete, completion is blocked and the transaction aborts.Other restricted instructions are handled similarly, including TBEGIN ifdecoded while in a constrained transaction, or exceeding the maximumnesting depth.

An outermost TBEGIN is cracked into multiple micro-ops depending on theGR-Save-Mask; each micro-op will be executed by one of the two fixedpoint units (FXUs) to save a pair of GRs into a specialtransaction-backup register file, that is used to later restore the GRcontent in case of a transaction abort. Also the TBEGIN spawns micro-opsto perform an accessibility test for the TDB if one is specified; theaddress is saved in a special purpose register for later usage in theabort case. At the decoding of an outermost TBEGIN, the instructionaddress and the instruction text of the TBEGIN are also saved in specialpurpose registers for a potential abort processing later on.

TEND and NTSTG are single micro-op instructions; NTSTG(non-transactional store) is handled like a normal store except that itis marked as non-transactional in the issue queue so that the LSU cantreat it appropriately. TEND is a no-op at execution time, the ending ofthe transaction is performed when TEND completes.

As mentioned, instructions that are within a transaction are marked assuch in the issue queue, but otherwise execute mostly unchanged; the LSUperforms isolation tracking as described in the next section.

Since decoding is in-order, and since the IDU keeps track of the currenttransactional state and writes it into the issue queue along with everyinstruction from the transaction, execution of TBEGIN, TEND, andinstructions before, within, and after the transaction can be performedout-of order. It is even possible (though unlikely) that TEND isexecuted first, then the entire transaction, and lastly the TBEGINexecutes. Program order is restored through the GCT at completion time.The length of transactions is not limited by the size of the GCT, sincegeneral purpose registers (GRs) can be restored from the backup registerfile.

During execution, the program event recording (PER) events are filteredbased on the Event Suppression Control, and a PER TEND event is detectedif enabled. Similarly, while in transactional mode, a pseudo-randomgenerator may be causing the random aborts as enabled by the TransactionDiagnostics Control.

Tracking for Transactional Isolation

The Load/Store Unit tracks cache lines that were accessed duringtransactional execution, and triggers an abort if an XI from another CPU(or an LRU-XI) conflicts with the footprint. If the conflicting XI is anexclusive or demote XI, the LSU rejects the XI back to the L3 in thehope of finishing the transaction before the L3 repeats the XI. This“stiff-arming” is very efficient in highly contended transactions. Inorder to prevent hangs when two CPUs stiff-arm each other, a XI-rejectcounter is implemented, which triggers a transaction abort when athreshold is met.

The L1 cache directory is traditionally implemented with static randomaccess memories (SRAMs). For the transactional memory implementation,the valid bits (64 rows×6 ways) of the directory have been moved intonormal logic latches, and are supplemented with two more bits per cacheline: the TX-read and TX-dirty bits.

The TX-read bits are reset when a new outermost TBEGIN is decoded (whichis interlocked against a prior still pending transaction). The TX-readbit is set at execution time by every load instruction that is marked“transactional” in the issue queue. Note that this can lead toover-marking if speculative loads are executed, for example on amispredicted branch path. The alternative of setting the TX-read bit atload completion time was too expensive for silicon area, since multipleloads can complete at the same time, requiring many read-ports on theload-queue.

Stores execute the same way as in non-transactional mode, but atransaction mark is placed in the store queue (STQ) entry of the storeinstruction. At write-back time, when the data from the STQ is writteninto the L1, the TX-dirty bit in the L1-directory is set for the writtencache line. Store write-back into the L1 occurs only after the storeinstruction has completed, and at most one store is written back percycle. Before completion and write-back, loads can access the data fromthe STQ by means of store-forwarding; after write-back, the CPU canaccess the speculatively updated data in the L1. If the transaction endssuccessfully, the TX-dirty bits of all cache-lines are cleared, and alsothe TX-marks of not yet written stores are cleared in the STQ,effectively turning the pending stores into normal stores.

On a transaction abort, all pending transactional stores are invalidatedfrom the STQ, even those already completed. All cache lines that weremodified by the transaction in the L1, that is, have the TX-dirty biton, have their valid bits turned off, effectively removing them from theL1 cache instantaneously.

The architecture requires that before completing a new instruction, theisolation of the transaction read- and write-set is maintained. Thisisolation is ensured by stalling instruction completion at appropriatetimes when XIs are pending; speculative out-of order execution isallowed, optimistically assuming that the pending XIs are to differentaddresses and not actually cause a transaction conflict. This designfits very naturally with the XI-vs-completion interlocks that areimplemented on prior systems to ensure the strong memory ordering thatthe architecture requires.

When the L1 receives an XI, L1 accesses the directory to check validityof the XI'ed address in the L1, and if the TX-read bit is active on theXI'ed line and the XI is not rejected, the LSU triggers an abort. When acache line with active TX-read bit is LRU′ed from the L1, a specialLRU-extension vector remembers for each of the 64 rows of the L1 that aTX-read line existed on that row. Since no precise address trackingexists for the LRU extensions, any non-rejected XI that hits a validextension row the LSU triggers an abort. Providing the LRU-extensioneffectively increases the read footprint capability from the L1-size tothe L2-size and associativity, provided no conflicts with other CPUsagainst the non-precise LRU-extension tracking causes aborts.

The store footprint is limited by the store cache size (the store cacheis discussed in more detail below) and thus implicitly by the L2 sizeand associativity. No LRU-extension action needs to be performed when aTX-dirty cache line is LRU′ed from the L1.

Store Cache

In prior systems, since the L1 and L2 are store-through caches, everystore instruction causes an L3 store access; with now 6 cores per L3 andfurther improved performance of each core, the store rate for the L3(and to a lesser extent for the L2) becomes problematic for certainworkloads. In order to avoid store queuing delays a gathering storecache had to be added, that combines stores to neighboring addressesbefore sending them to the L3.

For transactional memory performance, it is acceptable to invalidateevery TX-dirty cache line from the L1 on transaction aborts, because theL2 cache is very close (7 cycles L1 miss penalty) to bring back theclean lines. However, it would be unacceptable for performance (andsilicon area for tracking) to have transactional stores write the L2before the transaction ends and then invalidate all dirty L2 cache lineson abort (or even worse on the shared L3).

The two problems of store bandwidth and transactional memory storehandling can both be addressed with the gathering store cache. The cacheis a circular queue of 64 entries, each entry holding 128 bytes of datawith byte-precise valid bits. In non-transactional operation, when astore is received from the LSU, the store cache checks whether an entryexists for the same address, and if so gathers the new store into theexisting entry. If no entry exists, a new entry is written into thequeue, and if the number of free entries falls under a threshold, theoldest entries are written back to the L2 and L3 caches.

When a new outermost transaction begins, all existing entries in thestore cache are marked closed so that no new stores can be gathered intothem, and eviction of those entries to L2 and L3 is started. From thatpoint on, the transactional stores coming out of the LSU STQ allocatenew entries, or gather into existing transactional entries. Thewrite-back of those stores into L2 and L3 is blocked, until thetransaction ends successfully; at that point subsequent(post-transaction) stores can continue to gather into existing entries,until the next transaction closes those entries again.

The store cache is queried on every exclusive or demote XI, and causesan XI reject if the XI compares to any active entry. If the core is notcompleting further instructions while continuously rejecting XIs, thetransaction is aborted at a certain threshold to avoid hangs.

The LSU requests a transaction abort when the store cache overflows. TheLSU detects this condition when it tries to send a new store that cannotmerge into an existing entry, and the entire store cache is filled withstores from the current transaction. The store cache is managed as asubset of the L2: while transactionally dirty lines can be evicted fromthe L1, they have to stay resident in the L2 throughout the transaction.The maximum store footprint is thus limited to the store cache size of64×128 bytes, and it is also limited by the associativity of the L2.Since the L2 is 8-way associative and has 512 rows, it is typicallylarge enough to not cause transaction aborts.

If a transaction aborts, the store cache is notified and all entriesholding transactional data are invalidated. The store cache also has amark per doubleword (8 bytes) whether the entry was written by a NTSTGinstruction—those doublewords stay valid across transaction aborts.

Millicode-Implemented Functions

Traditionally, IBM mainframe server processors contain a layer offirmware called millicode which performs complex functions like certainCISC instruction executions, interruption handling, systemsynchronization, and RAS. Millicode includes machine dependentinstructions as well as instructions of the instruction set architecture(ISA) that are fetched and executed from memory similarly toinstructions of application programs and the operating system (OS).Firmware resides in a restricted area of main memory that customerprograms cannot access. When hardware detects a situation that needs toinvoke millicode, the instruction fetching unit switches into “millicodemode” and starts fetching at the appropriate location in the millicodememory area. Millicode may be fetched and executed in the same way asinstructions of the instruction set architecture (ISA), and may includeISA instructions.

For transactional memory, millicode is involved in various complexsituations. Every transaction abort invokes a dedicated millicodesub-routine to perform the necessary abort steps. The transaction-abortmillicode starts by reading special-purpose registers (SPRs) holding thehardware internal abort reason, potential exception reasons, and theaborted instruction address, which millicode then uses to store a TDB ifone is specified. The TBEGIN instruction text is loaded from an SPR toobtain the GR-save-mask, which is needed for millicode to know which GRsto restore.

The CPU supports a special millicode-only instruction to read out thebackup-GRs and copy them into the main GRs. The TBEGIN instructionaddress is also loaded from an SPR to set the new instruction address inthe PSW to continue execution after the TBEGIN once the millicode abortsub-routine finishes. That PSW may later be saved as program-old PSW incase the abort is caused by a non-filtered program interruption.

The TABORT instruction may be millicode implemented; when the IDUdecodes TABORT, it instructs the instruction fetch unit to branch intoTABORT's millicode, from which millicode branches into the common abortsub-routine.

The Extract Transaction Nesting Depth (ETND) instruction may also bemillicoded, since it is not performance critical; millicode loads thecurrent nesting depth out of a special hardware register and places itinto a GR. The PPA instruction is millicoded; it performs the optimaldelay based on the current abort count provided by software as anoperand to PPA, and also based on other hardware internal state.

For constrained transactions, millicode may keep track of the number ofaborts. The counter is reset to 0 on successful TEND completion, or ifan interruption into the OS occurs (since it is not known if or when theOS will return to the program). Depending on the current abort count,millicode can invoke certain mechanisms to improve the chance of successfor the subsequent transaction retry. The mechanisms involve, forexample, successively increasing random delays between retries, andreducing the amount of speculative execution to avoid encounteringaborts caused by speculative accesses to data that the transaction isnot actually using. As a last resort, millicode can broadcast to otherCPUs to stop all conflicting work, retry the local transaction, beforereleasing the other CPUs to continue normal processing. Multiple CPUsmust be coordinated to not cause deadlocks, so some serializationbetween millicode instances on different CPUs is required.

FIG. 4 is a flowchart depicting a method for monitoring the amount ofavailable resource for transactional execution and determining if thereis enough available resource for transactional execution to commit ahardware transaction, in accordance with embodiments of the presentdisclosure. Before discussing the particulars of FIG. 4, a generaldiscussion of embodiments of the present disclosure will be had.

As previously described, transactional semantics in transactional memorysystems may be enforced by tracking the memory locations read andwritten by each transaction. If multiple transactionally executinglogical processors access the same memory in a conflicting way, one ormore of the competing (i.e., conflicting) transactions may be aborted.For example, two accesses of the same memory location may be conflictingif at least one of the accesses is a write. Similarly, if anotherlogical (or real) processor accesses a memory location of atransactional memory of a processor executing a transaction, a conflictmay be detected and the transaction may be aborted.

Transactional memory systems may leverage a cache coherence protocol toenforce transactional semantics, using cache lines to detect transactionconflicts. For example, each cache line may be associated withtransactional access bits in addition to the valid bit, coherence statebits, and other descriptive bits that may be associated with the cacheline for maintaining cache coherence and for other various uses. Atransactional read bit (R) may be added to indicate whether a cache linehas been read during execution of a transaction. A transactional writebit (W) may be added to indicate whether a cache line has been writtenduring execution of a transaction.

As further previously described, transactional memory systems implementtransactional semantics through a variety of techniques. Generally, agiven transactional memory system will include a mechanism formaintaining speculative state for a transaction that is being executed(e.g., speculative state may be maintained in-place, while rollbackinformation is preserved in an undo-log; or speculative state may bemaintained in a write buffer, while rollback information is preservedin-place, etc.). If one or more executing transactions exhausts aresource dedicated to the maintenance of speculative state, one or moreof the transactions may be aborted. For example, a transactional memorysystem that includes a hard limit on the number of nested transactionsmay abort one, some, or all nested transactions if the number of nestedtransactions exceeds the hard limit. For another example, atransactional memory system that includes a hard limit on the amount ofstored speculative state may abort one or more transactions if theamount of speculative state exceeds the hard limit (which may include:available cache space, cache lines, cache set associativity, storebuffer size, available store buffer entries, nesting level limits, orthe hard limit on the amount of speculative state may be based on, e.g.,the size of the write buffer that maintains speculative state, etc.).

According to embodiments of the present disclosure, upon detecting thatthe amount of available resource for transactional execution isexhausted or approaching exhaustion during an executing hardwaretransaction, instead of aborting the executing transaction, thetransactional memory system can salvage the partially executedtransaction by jumping to and executing an about-to-run-out-of-resourcehandler. An about-to-run-out-of-resource handler determines if thespeculative state is salvageable and if so, commits any salvageablespeculative state. By salvaging the partially executed transaction, thetransactional memory system thus preserves and utilizes speculativestate that otherwise would have been discarded upon abort.

In an exemplary embodiment, when the amount of available resource fortransactional execution, such as nesting level, cache space or bufferspace, is about to run out during an executing hardware transaction,hardware may transfer control to an about-to-run-out-of-resource handlerwith an indication that the transaction is about to run out of resourcefor transactional execution. Then the about-to-run-out-of-resourcehandler may determine if there is salvageable transaction speculativestate, and if there is, may commit stores of the partially executedtransaction to memory. Further, hardware, such asabout-to-run-out-of-resource handler information determiner 911 in FIG.9, may provide a means to software to indicate the address of anabout-to-run-out-of-resource handler. This can be done by providing anexplicit instruction to indicate such an address (e.g., arecord-about-to-run-out-of-resource-handler-address instruction, etc.).Alternatively, hardware can use the same address for theabout-to-run-out-of-resource-handler (i.e., in an embodiment that hasboth a run-out-of-resource handler and an about-to-run-out-of-resourcehandler) and provide an indicator in, e.g., in a condition register,(e.g., salvaging register(s) 910, etc.), etc., that the transaction isabout to run out of resource but has not yet actually run out ofresource. Information regarding the about-to-run-out-of-resourcehandler, such as an instruction address (program counter address) of anext transaction instruction, can be stored by the transactional memorysystem in a register (e.g., salvaging register(s) 910, etc.). Hardware,for example, available resource detector 914 of FIG. 9, detects when atransaction is about to run out of resource and transfers, for example,using hardware transaction transferor 915, control to theabout-to-run-out-of-resource handler.

Software can use these techniques as follows to salvage computation inhardware transactions: Software can check a condition register todetermine if the current transaction is about to exhaust the availableresource for transactional execution or if the current transaction hascaused the amount of available resource to drop below a threshold level.In the about-to-run-out-of-resource handler, software determines if anyof the partially executed transaction is salvageable, and commits anysalvageable speculative state. Software may proceed to execute theremaining instructions in the transaction necessary to bring thetransaction to a stable state, and upon completion, may commit part orall of the speculative state.

In other embodiments, in which an allowable amount (limit) of nestinglevels is the amount of available resource for transactional execution,the about-to-run-out-of-resource handler may determine that the currenttransaction is about to exceed the nesting level limit, by for example,looking ahead at the next instruction and determining that execution ofthe next instruction would result in the nesting level limit beingexceeded. In this embodiment, the about-to-run-out-of-resource handlerthen determines if any of the partially executed transaction issalvageable, and commits any salvageable speculative state. In evenfurther embodiments, if the about-to-run-out-of-resource handlerdetermines the current transaction is about to exhaust the availableresource for transactional execution, which in this embodiment isavailable cache lines for transactional execution, theabout-to-run-out-of-resource handler may commit data in a specificamount of cache lines to memory in order to free up additional cachelines for use in execution of the current transaction.

In the exemplary embodiment, after the transaction memory system(hardware) has detected that the amount of available resource fortransactional execution, such as nesting level cache space or storebuffer space, is below a certain threshold percentage, the transactionalmemory system may transfer control to an about-to-run-out-of-resourcehandler (software) (step 402). In an exemplary embodiment, the certainthreshold percentage may be 10%. In other embodiments, the certainthreshold percentage may be another value or expressed in other terms,such as available cache, store buffer or nesting levels rather than apercentage. In even further embodiments, rather than detecting thatavailable resource for transactional execution has crossed below thethreshold, the transactional memory system may detect that exhaustion ofthe available resource for transactional execution is impending. Forexample, transactional memory system may detect that execution of apending instruction in the transaction will cause the available resourcefor transactional execution to fall below the threshold level or exhaustthe available resource. In this case, transactional memory system stopsthe execution of the instruction, therefore preventing the transactionfrom causing the available resource for transactional execution to fallbelow the threshold level or exhaust the available resource fortransactional execution, and transfers control to theabout-to-run-out-of-resource handler. In yet another embodiment,transactional memory system may detect that execution of a pendinginstruction will cause the hard limit on the number of nestedtransaction to be exceeded. In the exemplary embodiment, theabout-to-run-out-of-resource handler is a software application capableof determining whether or not a transaction can salvaged. In anotherembodiment, the about-to-run-out-of-resource handler may be executed bya supervisor, by context switching without leaving the transaction mode.

Once control is transferred over to the about-to-run-out-of-resourcehandler, the about-to-run-out-of-resource handler determines if at leasta portion of the relevant transaction can be salvaged and committedsafely to memory (decision 404). In the exemplary embodiment, theabout-to-run-out-of-resource handler reads memory locations to determinehow much computation was executed in the transaction. The reads may beof memory locations already accessed by the transaction, thus notrequiring the transaction to use additional available resource. Based onthe read values, the about-to-run-out-of-resource handler thendetermines the amount of computation needed to bring the transaction toa stable state and whether there is enough available resource fortransactional execution to bring the transaction to a stable state. Ifthe about-to-run-out-of-resource handler determines that the transactioncannot be salvaged (i.e., buffer space freed-up or nesting levels heldwithin limit for example) (decision 404, “NO” branch), theabout-to-run-out-of-resource handler may issue an abort instructioncausing the transaction to fail (step 406) or return to the transactionto allow it to fail. For example, a “TABORT” or “XABORT” instruction, orgenerally, an “abort_transaction” instruction may be issued. If theabout-to-run-out-of-resource handler determines that the transaction canbe salvaged (decision 404, “YES” branch), theabout-to-run-out-of-resource handler may execute instructions needed totake the transaction to a stable state and issue a transaction endinstruction to commit the transaction to memory (step 408). For example,a partially executed transaction is committed by issuing a “TEND” or“XEND” instruction, or generally, a “commit_transaction” instruction.The about-to-run-out-of-resource handler may also issue a TBEGIN tocontinue executing the remainder of the transaction transactionally, ormay acquire a lock to allow the remainder of the transaction to completenon-transactionally for example.

For example, a hardware transaction may involve copying a matrix A intoa matrix B, with the requirement that individual rows need to be updatedatomically. In this example, if during the execution of the transaction,the transaction memory system determines that the amount of availableresource for transactional execution is below the certain thresholdpercentage or is exhausted, transactional memory system determines ifthe transaction can be committed safely to memory. In order for thetransaction to be committed safely, transaction memory system must bringthe transaction to a stable state. In this example, since only rows needto be updated atomically, transaction memory system can bring thetransaction to a stable state by completing the current row beingcopied. Transaction memory system determines if there is enoughavailable resource for transactional execution to bring the transactionto a stable state and partially commit the stable transaction to memory.If there is enough available resource to bring the transaction to astable state, transaction memory system may bring the transaction to astable state and perform a partial commit-to-memory of the transaction.Then issue a TBEGIN to continue executing the transaction. If there isnot enough available resource to bring the transaction to a stablestate, transaction memory system may cause the transaction to fail(abort).

The about-to-run-out-of-resource handler then may execute code orinstructions to compensate for the incomplete portion of the salvagedtransaction. This can be done non-transactionally or using one or morenew transactions for example.

In other embodiments, once control is transferred to theabout-to-run-out-of-resource handler, the about-to-run-out-of-resourcehandler may attempt to bring the transaction to a stable statetransaction without first determining if there is enough availableresource for transactional execution. In this embodiment, if there isnot enough resource for transactional execution, the transactionalmemory system will cause the transaction to fail. If there is enoughresource for transactional execution, transactional memory system willexecute the transaction (i.e., bring the transaction to a stable state)and commit the transaction to memory.

FIG. 5 illustrates an embodiment of the present disclosure fordetermining whether a hardware transaction can be salvaged. Theembodiment executes a transaction in a transactional memory environment(step 502). Based on detecting that the amount of available resource isbelow a predetermined threshold level, the embodiment determines whetherthe transaction can be salvaged (decision 504). If the transaction cannot be salvaged (decision 504, “NO” branch), the embodiment aborts thetransaction (step 506). If the transaction can be salvaged (decision504, “YES” branch), the embodiment performs a salvage operation, withthe salvage operation comprising one or more of: 1) determining that thetransaction can be brought to a stable state without exceeding theamount of available resource for transactional execution and bringingthe transaction to a stable state; and 2) determining that a resourcecan be made available, and making the resource available (step 508). Theembodiment commits the salvaged transaction (step 510).

As depicted in FIG. 6, the illustrated embodiment makes the resourceavailable (block 602) and further performs one of the following:continues execution of the transaction (block 604); ends and restartsexecution of the transaction (block 606); obtains a lock and continuesexecution of the transaction in non-transactional execution mode (block608); or continues execution of the transaction in non-transactionexecution mode (block 610). Additionally, as depicted in FIG. 7, theillustrated embodiment determines that the transaction can be brought toa stable state without exceeding the amount of available resource fortransactional execution (block 702) based on determining the amount ofcomputation executed in the transaction based on read values (block704). As depicted in FIG. 8, the illustrated embodiment brings thetransaction to a stable state (block 802) by executing one or moreinstructions to compensate for an incomplete portion of the salvagedtransaction (block 804).

Referring now to FIG. 9, a functional block diagram of a computer systemin accordance with an embodiment of the present disclosure is shown.Computer system 900 is only one example of a suitable computer systemand is not intended to suggest any limitation as to the scope of use orfunctionality of embodiments of the disclosure described herein.Regardless, computer system 900 is capable of being implemented and/orperforming any of the functionality set forth hereinabove.

In computer system 900 there is computer 912, which is operational withnumerous other general purpose or special purpose computing systemenvironments or configurations. Examples of well-known computingsystems, environments, and/or configurations that may be suitable foruse with computer 912 include, but are not limited to, personal computersystems, server computer systems, thin clients, thick clients, handheldor laptop devices, multiprocessor systems, microprocessor-based systems,set top boxes, programmable consumer electronics, network PCs,minicomputer systems, mainframe computer systems, and distributed cloudcomputing environments that include any of the above systems or devices,and the like.

Computer 912 may be described in the general context of computer systemexecutable instructions, such as program modules, being executed by acomputer system. Generally, program modules may include routines,programs, objects, components, logic, data structures, and so on thatperform particular tasks or implement particular abstract data types.Computer 912 may be practiced in distributed cloud computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed cloudcomputing environment, program modules may be located in both local andremote computer system storage media including memory storage devices.

As further shown in FIG. 9, computer 912 in computer system 900 is shownin the form of a general-purpose computing device. The components ofcomputer 912 may include, but are not limited to, one or more processorsor processing units 916, memory 928, and bus 918 that couples varioussystem components including memory 928 to processing unit 916.Processing units 916 can, in various embodiments, include some or all ofCPU 114 a, 114 b of FIG. 1, transactional CPU environment 112 of FIG. 2,and the processor of FIG. 3. Further, processing units 916 can, invarious embodiments, include about-to-run-out-of-resource handlerinformation determiner 911, some or all of salvaging register(s) 910,available resource detector 914, and hardware transaction transferor915, as discussed above in the context of FIG. 4.

Bus 918 represents one or more of any of several types of busstructures, including a memory bus or memory controller, a peripheralbus, an accelerated graphics port, and a processor or local bus usingany of a variety of bus architectures. By way of example, and notlimitation, such architectures include Industry Standard Architecture(ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA)bus, Video Electronics Standards Association (VESA) local bus, andPeripheral Component Interconnect (PCI) bus.

Computer 912 typically includes a variety of computer system readablemedia. Such media may be any available media that is accessible bycomputer 912, and includes both volatile and non-volatile media, andremovable and non-removable media.

Memory 928 can include computer system readable media in the form ofvolatile memory, such as random access memory (RAM) 930 and/or cache932. In one embodiment, cache 932, and/or additional caches, areincluded in processing unit 916. Computer 912 may further include otherremovable/non-removable, volatile/non-volatile computer system storagemedia. By way of example only, computer readable storage media 934 canbe provided for reading from and writing to a non-removable,non-volatile magnetic media (not shown and typically called a “harddrive”). Although not shown, a magnetic disk drive for reading from andwriting to a removable, non-volatile magnetic disk (e.g., a “floppydisk”), and an optical disk drive for reading from or writing to aremovable, non-volatile optical disk such as a CD-ROM, DVD-ROM or otheroptical media can be provided. In such instances, each can be connectedto bus 918 by one or more data media interfaces. As will be furtherdepicted and described below, memory 928 may include at least oneprogram product having a set (e.g., at least one) of program modulesthat are configured to carry out some or all of the functions ofembodiments of the disclosure.

Program 940, having one or more program modules 942, may be stored inmemory 928 by way of example, and not limitation, as well as anoperating system, one or more application programs, other programmodules, and program data. Each of the operating system, one or moreapplication programs, other program modules, and program data or somecombination thereof, may include an implementation of a networkingenvironment. In some embodiments, program modules 942 generally carryout the functions and/or methodologies of embodiments of the disclosureas described herein, while in other embodiments, processing unit 916generally carries out the functions and/or methodologies of embodimentsof the disclosure as described herein, while in yet other embodimentsother portions of computer 912 generally carry out the functions and/ormethodologies of embodiments of the disclosure as described herein.

Computer 912 may also communicate with one or more external devices 913such as a keyboard, a pointing device, etc., as well as display 924; oneor more devices that enable a user to interact with computer 912; and/orany devices (e.g., network card, modem, etc.) that enable computer 912to communicate with one or more other computing devices. Suchcommunication can occur via Input/Output (I/O) interfaces 922. Stillyet, computer 912 can communicate with one or more networks such as alocal area network (LAN), a general wide area network (WAN), and/or apublic network (e.g., the Internet) via network adapter 920. Asdepicted, network adapter 920 communicates with the other components ofcomputer 912 via bus 918. It should be understood that although notshown, other hardware and/or software components could be used inconjunction with computer 912. Examples include, but are not limited to:microcode, device drivers, redundant processing units, external diskdrive arrays, RAID systems, tape drives, and data archival storagesystems, etc.

Various embodiments of the disclosure may be implemented in a dataprocessing system suitable for storing and/or executing program codethat includes at least one processor coupled directly or indirectly tomemory elements through a system bus. The memory elements include, forinstance, local memory employed during actual execution of the programcode, bulk storage, and cache memory which provide temporary storage ofat least some program code in order to reduce the number of times codemust be retrieved from bulk storage during execution.

Input/Output or I/O devices (including, but not limited to, keyboards,displays, pointing devices, DASD, tape, CDs, DVDs, thumb drives andother memory media, etc.) can be coupled to the system either directlyor through intervening I/O controllers. Network adapters may also becoupled to the system to enable the data processing system to becomecoupled to other data processing systems or remote printers or storagedevices through intervening private or public networks. Modems, cablemodems, and Ethernet cards are just a few of the available types ofnetwork adapters.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The flow diagrams depicted herein are just examples. There may be manyvariations to these diagrams or the operations described therein withoutdeparting from the spirit of the disclosure. For instance, theoperations may be performed in a differing order, or operations may beadded, deleted, or modified. All of these variations are considered apart of the claimed disclosure.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the disclosure, and these are,therefore, considered to be within the scope of the disclosure, asdefined in the following claims.

What is claimed is:
 1. A computer system for determining whether ahardware transaction can be salvaged, the computer system comprising: amemory; and a processor in communication with the memory, wherein thecomputer system is configured to perform a method, said methodcomprising: beginning, by the processor, execution of a transaction in atransactional memory environment; based on detection that an amount ofavailable resource for transactional execution is below a predeterminedthreshold level, determining, by the processor, whether the transactioncan be salvaged; based on determining that the transaction can not besalvaged, aborting, by the processor, the transaction; and based ondetermining the transaction can be salvaged, performing, by theprocessor, a salvage operation, wherein the salvage operation comprisesone or more of: a) determining, by the processor, that the transactioncan be brought to a stable state without exceeding the amount ofavailable resource for transactional execution, and bringing thetransaction to a stable state; and b) determining, by the processor,that a resource can be made available, and making the resourceavailable.
 2. The computer system of claim 1, further comprising, basedon determining the transaction can be salvaged, committing, by theprocessor, the salvaged transaction to memory.
 3. The computer system ofclaim 1, wherein based on making the resource available, said methodfurther comprising one of: continuing execution of the transaction,ending and restarting execution of the transaction, obtaining a lock andcontinuing execution of the transaction in non-transactional executionmode, or continuing execution of the transaction in non-transactionalexecution mode.
 4. The computer system of claim 1, wherein the bringingthe transaction to a stable state further comprises executing, by theprocessor, one or more instructions to compensate for an incompleteportion of the salvaged transaction.
 5. The computer system of claim 1,wherein the determining that the transaction can be brought to a stablestate without exceeding the amount of available resource fortransactional execution includes determining, by the processor, theamount of computation executed in the transaction based on read values.6. A computer program product for determining whether a hardwaretransaction can be salvaged, the computer program product comprising: acomputer readable storage medium readable by a processing circuit andstoring instructions for execution by the processing circuit forperforming a method comprising: beginning, by a processor, execution ofa transaction in a transactional memory environment; based on detectionthat an amount of available resource for transactional execution isbelow a predetermined threshold level, determining, by the processor,whether the transaction can be salvaged; based on determining that thetransaction can not be salvaged, aborting, by the processor, thetransaction; and based on determining the transaction can be salvaged,performing, by the processor, a salvage operation, wherein the salvageoperation comprises one or more of: c) determining, by the processor,that the transaction can be brought to a stable state without exceedingthe amount of available resource for transactional execution, andbringing the transaction to a stable state; and d) determining, by theprocessor, that a resource can be made available, and making theresource available.
 7. The computer program product of claim 6, furthercomprising, based on determining, by the processor, the transaction canbe salvaged, committing the salvaged transaction to memory.
 8. Thecomputer program product of claim 6, wherein based on making theresource available, said method further comprising one of: continuingexecution of the transaction, ending and restarting execution of thetransaction, obtaining a lock and continuing execution of thetransaction in non-transactional execution mode, or continuing executionof the transaction in non-transactional execution mode.
 9. The computerprogram product of claim 6, wherein the bringing the transaction to astable state further comprises executing, by the processor, one or moreinstructions to compensate for an incomplete portion of the transactioncommitted to memory.
 10. The computer program product of claim 6,wherein the determining that the transaction can be brought to a stablestate without exceeding the amount of available resource fortransactional execution includes determining, by the processor, theamount of computation executed in the transaction based on read values.11. A method for determining whether a hardware transaction can besalvaged, the method comprising: beginning execution, by a processor, ofa transaction in a transactional memory environment; based on detectionthat an amount of available resource for transactional execution isbelow a predetermined threshold level, determining, by the processor,whether the transaction can be salvaged; based on determining that thetransaction can not be salvaged, aborting, by the processor, thetransaction; and based on determining the transaction can be salvaged,performing, by the processor, a salvage operation, wherein the salvageoperation comprises one or more of: e) determining, by the processor,that the transaction can be brought to a stable state without exceedingthe amount of available resource for transactional execution, andbringing the transaction to a stable state; and f) determining, by theprocessor, that a resource can be made available, and making theresource available.
 12. The method of claim 11, further comprising,based on determining the transaction can be salvaged, committing, by theprocessor, the salvaged transaction to memory.
 13. The method of claim11, further comprising: based on making the resource available, themethod further comprising one of: continuing execution of thetransaction, ending and restarting execution of the transaction,obtaining a lock and continuing execution of the transaction innon-transactional execution mode, or continuing execution of thetransaction in non-transactional execution mode.
 14. The method of claim11, wherein the bringing the transaction to a stable state furthercomprises executing, by the processor, one or more instructions tocompensate for an incomplete portion of the transaction committed tomemory.
 15. The method of claim 11, wherein the determining that thetransaction can be brought to a stable state without exceeding theamount of available resource for transactional execution includesdetermining, by the processor, the amount of computation executed in thetransaction based on read values.